Personnel or asset location systems are routinely used in manufacturing and hospital environments. However, with increasing concerns for security and inventory control, such systems have progressed beyond typical manufacturing and hospital environments to other areas such as, for example, government and corporate environments.
In order to provide accurate location of the tracked object, a transmitter is commonly attached to the object. The transmitter stores information regarding the object's characteristic and routinely, or upon prompting, using various transmission methodologies transmits, for example, radio, ultrasonic, or optical signals to various receivers located within the tracking environment. Based on either time-difference of arrival information or other methods a locator system can be developed. However, problems with reliability, power consumption, signal interference, and the like have plagued conventional systems.
For example, location systems using radio frequency transmitters are described in U.S. Pat. No. 6,104,295 by Gaisser et al., for tracking of hospital patients. The Gaisser system has the disadvantage that it requires, in addition to the radio frequency (RF) signal, an infrared signal to accurately locate objects within the tracking environment. Also, RF waves are very directional and in steel buildings require multiple receivers due to reflections and ghosting. Also radio frequency signals are easily contaminated with ambient electronic noise such as motors starting, fluorescent lights, computers, and low power communication systems.
U.S. Pat. No. 5,027,314 to Linwood et al. discloses a tracking system using a light based transmitter, and U.S. Pat. No. 5,572,195 to Heller et al. uses light, such as infrared transmitted signals, to determine the location of a tracked object. However, infrared based location systems suffer from narrow beam widths and beam blockage. Also, infrared radiation is known to go through glass windows and, therefore, transmitters using infrared radiation can “leak” into other rooms or locations resulting in false locations.
U.S. Pat. No. 3,439,320 to Ward describes a personnel location system using ultrasonic transducers using unique frequencies to identify the tracked person. U.S. Pat. No. 3,805,265 to Lester discloses a location system using any one of radiant waves such as light waves, acoustic waves, ultrasonic waves, or microwaves, which are carried and transmitted from a central console to track a particular person. However, Lester describes a chained network of transducers conveying location signals from one cell to the next to arrive at the central console. Multiple frequencies are used to distinguish each transmitter from member transmitters, the transmitters acting as transceivers for each link in the chain.
Despite the various methods used above for tracking or locating objects within an environment, each of the above systems suffer from disadvantages such as for example, an excessive size, short battery life, interference, mislocation, and/or false location reads. Therefore, there has been a long standing need in the tracking field for a locating system that at least to some extent overcomes the limitations described above.